1. Field of the Invention
The present invention relates to a surface potential sensor and, more particularly, to the construction of a processing circuit for processing an electric signal from a fork of a surface potential sensor.
2. Description of Related Art
In general, a surface potential sensor is constructed with a chopper portion for chopping electric lines of force emitted from an objective to be measured at a constant frequency, an electrode for deriving the electric lines of force chopped by the chopper portion as an A.C. signal and a pre-amplifier for amplifying this A.C. signal and converting this impedance. A device of this type is described in Japanese Patent Appln. Laid-Open No. S60-231176.
In use of the surface potential sensor constructed as mentioned above, an externally provided fork oscillator circuit is connected between the fork driving terminal and the detecting terminal so that the fork is driven at a resonance frequency.
In conventional surface potential sensors, noise is induced in the detection electrode through a space of the sensor due to a signal supplied to the driving and detecting terminals. Since the detection electrode is kept at high impedance in order to allow it to pick up and amplify a very low amplitude signal, it is sensitive to noise from the driving and detecting terminals. Such noise may become an offset output of the surface potential sensor and, therefore, an A.C. voltage signal is detected, even when the electric lines of force are cut and the measured voltage should be OV.
Since the offset output has a frequency component equal to that of the detection signal, it is impossible to remove the offset even if a frequency filter circuit is added. Also, there is a deviation between input/output characteristics when a low voltage measurement is performed and, it makes an accurate measurement difficult.